I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for frequency tracking of multipath signals which have been subjected to doppler shifts.
II. Description of the Related Art
Frequency tracking loops are commonly used in direct sequence spread spectrum communication systems such as that described in the IS-95 over the air interface standard and its derivatives such as IS-95-A and ANSI J-STD-008 (referred to hereafter collectively as the IS-95 standard) promulgated by the Telecommunication Industry Association (TIA) and used primarily within cellular telecommunications systems. The IS-95 standard incorporates code division multiple access (CDMA) signal modulation techniques to conduct multiple communications simultaneously over the same RF bandwidth. When combined with comprehensive power control, conducting multiple communications over the same bandwidth increases the total number of calls and other communications that can be conducted in a wireless communication system by, among other things, increasing the frequency reuse in comparison to other wireless telecommunication technologies. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both of which are assigned to the assignee of the present invention and incorporated by reference herein.
FIG. 1 provides a highly simplified illustration of a cellular telephone system configured in accordance with the use of the IS-95 standard. During operation, a set of subscriber units 10a-d conduct wireless communication by establishing one or more RF interfaces with one or more base stations 12a-dusing CDMA modulated RF signals. Each RF interface between a base station 12 and a subscriber unit 10 is comprised of a forward link signal transmitted from the base station 12, and a reverse link signal transmitted from the subscriber unit. Using these RF interfaces, a communication with another user is generally conducted by way of mobile telephone switching office (MTSO) 14 and public switch telephone network (PSTN) 16. The links between base stations 12, MTSO 14 and PSTN 16 are usually formed via wire line connections, although the use of additional RF or microwave links is also known.
Each subscriber unit 10 communicates with one or more base stations 12 by utilizing a rake receiver. A RAKE receiver is described in U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated herein by reference. A rake receiver is typically made up of one or more searchers for locating direct and multipath pilot from neighboring base stations, and two or more fingers for receiving and combining information signals from those base stations. Searchers are described in co-pending U.S. patent application Ser. No. 08/316,177, entitled xe2x80x9cMULTIPATH SEARCH PROCESSOR FOR SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMSxe2x80x9d, filed Sep. 30, 1994, assigned to the assignee of the present invention and incorporated herein by reference.
In any passband digital communication system, such as that described above in relation to FIG. 1, there is a need for carrier synchronization. The sender modulates information onto a carrier at frequency fc, and the receiver must recover this frequency so that the received signal constellation does not rotate and degrade the signal to noise ratio (SNR) of the demodulated symbols. In the following discussion, the sender is a CDMA base station and the receiver is a CDMA subscriber unit.
Although the receiver knows the nominal carrier frequency, there are two main sources of error that contribute to the frequency difference between the received carrier from the base station and the carrier produced at the subscriber unit. First, the subscriber unit produces the carrier using a frequency synthesizer that uses a local clock as its timing reference. An example RF/IF section of a conventional heterodyne CDMA receiver is shown in FIG. 2. A signal received at antenna 200 is passed through low-noise amplifier (LNA) 202 and filtered in filter 204 before being mixed down to IF by RF mixer 206. This IF signal is filtered in filter 208, passed through variable-gain amplifier (VGA) 210 and is then mixed down to baseband by IF mixer 212. The baseband signal is then filtered in filter 214 and passed through analog to digital converter 216 to produce IQ symbols at baseband.
The carrier waveforms sent to RF and IF mixers 206 and 208 are produced using frequency synthesizers 218 and 220, respectively, that use the subscriber unit""s local clock as a timing reference. This clock has an unknown timing error, typically expressed in parts per million (ppm). In the exemplary implementation, this clock is voltage-controlled temperature compensated crystal oscillator (VCTCXO) 222, whose frequency is 19.68 MHz and is rated at +/xe2x88x925 ppm. This means if the desired waveform is a cellular 800 MHz carrier, the synthesized carrier applied to the RF mixer can be 800 MHz+/xe2x88x924000 Hz. Similarly, if the desired waveform is a 1900 MHz PCS carrier, the synthesized carrier can be 1900 MHz+/xe2x88x929500 Hz. To correct this error, CDMA receivers use a frequency tracking loop that monitors the frequency error and applies a tuning voltage to VCTCXO 222 to correct it.
The second source of error is due to frequency doppler created from movement of the subscriber unit station. The doppler effect manifests as an apparent change in the frequency of a received signal due to a relative velocity between the transmitter and receiver. The doppler contribution can be computed as       f    D    =                    v        λ            ⁢      cos      ⁢              xe2x80x83            ⁢      θ        =                  vf        c            ⁢      cos      ⁢              xe2x80x83            ⁢      θ      
where v is the velocity of the subscriber unit, x is the wavelength of the carrier, f is the carrier frequency, and c is the speed of light. The variable xcex8 represents the direction of travel of the subscriber unit relative to the direction of the received path from the base station. If the subscriber unit is travelling directly toward the base station, xcex8=0 degrees. If the subscriber unit is travelling directly away from the base station, xcex8=180 degrees. So the carrier frequency received at the subscriber unit changes depending on the speed and direction of the subscriber unit relative to the received signal path.
As mentioned above, CDMA systems use RAKE receivers that combine symbol energy from different paths. Each strong path is tracked by a finger that performs despreading, walsh decovering and accumulation, pilot time and frequency tracking, and symbol demodulation. An exemplary finger architecture is shown in FIG. 3, where each of N fingers 3A-3N outputs pilot and data symbols obtained for the path it is tracking to digital signal processor (DSP) 300. DSP 300 performs symbol demodulation and implements the time and frequency tracking loops. IQ baseband samples are despread in PN despreaders 310A-310N, and I and Q pilot and data samples are produced in walsh decover and accumulate blocks 320A-320N and 330A-330N, respectively.
An exemplary IS-95A CDMA receiver has four fingers to track four paths, whereas an exemplary cdma2OOO CDMA receiver has 12 fingers to handle the 3x multicarrier case. Cdma2OOO is described in TIA/EIA/IS-2000-2, entitled xe2x80x9cPHYSICAL LAYER STANDARD FOR CDMA2000 SPREAD SPECTRUM SYSTEMSxe2x80x9d, incorporated herein by reference. A subscriber unit can be tracking paths from different base stations (in soft handoff), as well as time-delayed paths from the same base station, created from reflections off of local objects. Since the angle xcex8 can be different for each path that the subscriber unit is tracking, the frequency doppler seen by each finger can be different, as illustrated in FIG. 4, which shows subscriber unit 400 in 3-way soft handoff with base stations 410A-410C. Subscriber unit 400 is traveling at velocity v and receiving signals from a variety of paths labeled Path 1 through Path 4. Path 1 comes from base station 410A at angle xcex81 equal to xcfx80. Path 2 comes from base station 410B at angle xcex82. Path 3 comes also comes from base station 410B but reflects off building 420 and arrives with angle xcex83. Path 4 comes from base station 410C and arrives with angle xcex84 equal to 0.
If we assume the subscriber unit has four fingers (labeled finger 1 through finger 4) and that finger i is tracking path i, we can see that the doppler seen by finger 1 is       -          vf      c        ,
the doppler seen by finger 2             vf      c        ⁢    cos    ⁢          xe2x80x83        ⁢          θ      2        ,
the doppler seen by finger 3 is             vf      c        ⁢    cos    ⁢          xe2x80x83        ⁢          θ      3        ,
and the doppler seen by finger 4 is       +          vf      c        ,
where v is the subscriber unit velocity, f is the carrier frequency, c is the speed of light, and xcex81 is the angle of incidence of the path with respect to the direction of subscriber unit 400.
To reduce the frequency error, CDMA receivers typically use a frequency locked loop that can be modeled as shown in FIG. 5. Frequency error detector 500 computes a measure of the difference between the received carrier frequency xcfx89(n) and the synthesized carrier frequency xcfx89(n). This error signal e(n) is filtered in loop filter 510 and fed back as c(n) to a voltage controlled oscillator (VCO) 520 that modifies the frequency of the synthesized carrier xcfx89(n). This closed-feedback loop corrects the carrier error.
We can apply this principle to a CDMA receiver as shown in FIGS. 6A and 6B. IQ baseband samples are passed into N fingers, labeled 600A-600N in FIG. 6B. FIG. 6A details the frequency error discrimination function of finger 600 which produces frequency error measure e(n). This functionality is replicated in fingers 600A-600N to produce frequency error measures e1(n)-eN(n), respectively. PN despreading and walsh accumulation to demodulate pilot symbols is performed in block 610. The resulting I(n) and Q(n) are delayed in blocks 620 and 630, respectively. The frequency error is measured by computing the phase rotation between successive pilot symbols in phase rotation measure block 640 to produce error measurement e(n).
Referring to FIG. 6B, frequency error measures e1(n)-eN(n) from fingers 600A-600N are added together in summer 650 and the sum is passed through loop filter 660 with adjustable gain xcex1. The result is sent to the voltage controlled oscillator 680 using pulse-density modulator (PDM) 670. Pulse density modulation is a method, known in the art, of converting a digital signal into an analog control voltage. This method applies a single frequency correction (by changing the local clock frequency) that affects all the fingers. In doing so, it basically neglects the individual doppler frequency error component affecting each finger.
As stated above, the frequency error has a local clock error component that is the same across all fingers, and a doppler component that is different across fingers. The conventional approach just discussed does not address the doppler component. Although the frequency doppler is not a serious problem at low speeds, it can become a problem when travelling at high speeds, such as on a bullet train. For bullet trains travelling at 500 km/hr, the maximum doppler is around 880 Hz, which can severely degrade the demodulated symbols and lead to dropped calls. So, for travelling on fast moving vehicles such as bullet trains, and in any other application where doppler effects on individual paths vary, there is a need for an improved frequency tracking loop in a CDMA receiver that considers the effect of doppler on each finger.
A novel and improved method and apparatus for frequency tracking is described. Frequency tracking is commonly utilized to provide for synchronization between locally generated carriers in a receiver and the carriers used at the base station to modulate the signals which are received. Two main sources of error that contribute to frequency difference include frequency offset between the two timing sources and doppler effects due to movement of a mobile receiver. In a CDMA system utilizing a RAKE receiver to demodulate multipath signals, each received multipath signal can contain a unique doppler effect as well as a common frequency offset component. The present invention provides a tracking mechanism for removing the effects of error due to frequency offset as well as compensation for frequency error due to doppler in a plurality of multipath signals.
Each finger of a RAKE receiver utilizing the present invention computes a frequency error for that finger. The weighted average of all of these frequency errors is calculated and filtered to provide a control signal for varying the frequency of IF and RF frequency synthesizers. This feature of the invention accounts for the common frequency offset seen at each finger.
Additionally, each finger is equipped with a rotator for providing frequency adjustment specific to that finger. The frequency of each finger is adjusted through feedback of the frequency error for that finger. One embodiment of the invention accomplishes this by subtracting the frequency error component of a finger from the overall weighted average, filtering the remainder, and with that filtered remainder controlling the rotator for that finger. In this way the weighted average of all the errors is used to drive the common frequency synthesis and the difference between the average and the specific error for each finger is used to drive each finger""s individual rotator and thus its doppler frequency compensation.
An alternative embodiment uses the independent frequency error for each finger directly by filtering it and using it to drive the rotator for that finger. Thus, the independent frequency errors are used to directly compensate for the doppler on each finger. These frequency errors, now doppler adjusted, are then weighted and averaged, the result of which is filtered and used to drive the frequency synthesizers. This weighted average accommodates the frequency offset component of the frequency error as well as the average of the doppler components of the various fingers. The gains of the individual loop filters can be adjusted in relation to the gain of the loop filter driving the frequency synthesizers so that the tracking speeds of the individual doppler compensation loops is appropriate in relation to the speed of the overall frequency offset tracking loop.
This alternative embodiment can be further refined to provide a system which ensures that the frequency synthesizers are tracking the average frequency error of all the fingers. In the previous embodiments, if the rotator loop compensates for some of the frequency error before its contribution is included in the synthesizer loop, the synthesizer loop may not be tracking the true weighted average. The refinement is to compute a second weighted average of the filtered versions of the individual frequency errors. This second weighted average is then multiplied by a factor and summed with the weighted average calculated as described above. The sum is used to drive the frequency synthesis loop. Therefore, even if the frequency error for a finger is driven to zero, effectively removing that finger""s contribution to the weighted average of frequency errors, its filtered frequency error will contribute to the second weighted average, and the synthesizer loop will be driven by it. Thus, the synthesizer loop will be driven according to the true weighted average of the finger frequency errors. Timing based on the average frequency error is a useful feature when used in other parts of a system. For example, a receiver""s timing may be useful for timing a transmitter to which it is coupled.